Add extension for RPC calls; replace locks; Refactored storage in Managed Client

Frameworks/MQTTnet.NetStandard/Server/MqttSubscriptionInterceptorContext.cs

@@ -1,17 +1,19 @@
-namespace MQTTnet.Server
+using System;
+
+namespace MQTTnet.Server
 {
     public class MqttSubscriptionInterceptorContext
     {
         public MqttSubscriptionInterceptorContext(string clientId, TopicFilter topicFilter)
         {
             ClientId = clientId;
-            TopicFilter = topicFilter;
+            TopicFilter = topicFilter ?? throw new ArgumentNullException(nameof(topicFilter));
         }
 
         public string ClientId { get; }
 
         public TopicFilter TopicFilter { get; }
-
+        
         public bool AcceptSubscription { get; set; } = true;
 
         public bool CloseConnection { get; set; }